Opaque vitreous enamels



Patented Apr. 30, 1946 UNITED, STATES PATENT or OPAQUE VITREOUS ENAMELS Ignaz Kreidl and Wcrner Kreidl,

Cleveland, Ohio Y No Drawing. Application March Serial N0. 383,794.

9 Claims. (01. 106-46) This invention relates to a process for the preparation of gas opacifled enamels which are prepared from an enamelv mass without the use of a suspension agent onmore particularly to vitreous enamels prepared by the dry process, or glass enamels and glazes, and the like.

e In the conventional process for the preparation of gas opacified enamels by the wet process it was customary to use gas opacifying substances which were absorbed on the suspension agent for the enamel slip such as especially gas opacifying substances absorbed on clay. In this way it was possible to have the gasopacifying substance finely distributed throughout the enamel slip and to regulate its gasification.' In contradistinction to the customary wet process, dry process enamelsand most glass enamels and glazes are prepared without the use of a suspension agent such as clay. It need not be mentioned that for the dry process enamels a suspension agent is not necessary since the ground enamel is dusted in the dry state onto the heated surface of the ware to be enameled. In the case of glass enamels or glazes it is customary to prepare the slip by finely grinding the enamel frit or glaze frit and to suspend it directly in a vehicle such as more particularly in alcohol or other organic vehicles, uch as oils, varnish bases or the like. I p

It is known that gas. opacifiers can not be melted into the frit since at the temperatures prevailing at and in the time necessary for the melting of the hit it would be impossible to retain a sumclent amount oi. gas bubbles in a sufficiently fine size and distribution in the frit. Accordingly, regardless of the type of enamel or glaze for which gas opacification is to be used the gas opacifying substance always will have to be added to the mill or will have to be mixed otherwise with the hit and if desired also with other additions. In order to utilize gas opaciflcation for such enamels or glazes which are presuspension agent when incorporating the gas opacifying substance in a material capable of distributing it in its pores so as to separate and subdivide the gas opacifying substance more or less independently of the fineness of the frit or glaze or other additions in the enamel mass. It should be noted that by"introducing into the enamel mass or glaze it is to be understood that the gas opacifying substance which is incorporated in the pores of the enclosing material is to be mixed I sary with other'additions.

pared without the use of a suspension agent it has been suggested to introduce the gas opacifying substance in a finely distributed way by introducing a sufiicient amount of clay having the gas opacifying substance absorbed thereon. It was found that although satisfactory opacity could be obtained in this way the clay produced unfavorable effects on the finished enamels, such as a porous surface, or matte surface and the like.

An object of the present invention is to provide a method for introducing gas opacifier into enamels of the type dispensing with the use of suspension agents in a. way which warrants even and line distribution thereof. Another object of with the enamel frit orfglaze frit and if neces- Thus, for example, the opacifying substance incorporated in the pores may be added to the mill together with the other mill additions such as the enamel or glaze frit.

It was found that more particularly extremely porous substances such as gels or gel-like substances and especially such inorganic substances will be suitable ,for the introduction of the gas opaciiying substance into the enamel mass. PreiL.

erably substances will be selected which in the dry state without the gas opacifying substance being incorporated therein have pores of a submicroscopic size that is smaller than about 10- cm'. in diameter and it will be also of great advantage if such substances contain at least about 70% by volume air. Furthermore it will be advantageous if such enclosing substances will retain their structure up to fairly high temperatures during the firing process. More particularly such inorganic substances will be of advantage since they will aid in providing the permanent network for the opacifying gas bubbles which are developed on firing of the enamel.

Especially suitable for the method according to the present invention, therefore, will be substances of the type of aerogels. Aerogels are known for their fine porosity and the extremely high air volume they contain. It is well known to those skilled in the art how such aerogels may be prepared and the preparation of such aerogels does not form subject of the present invention. Just by way of illustration it may be mentioned obtained as i'rom certain. borosilicate 90% silica. Y r

outwith mineral acids and thus a highly porous a porous glass appears the microscope an any of the above that such aerogels maybe prepared by gradually replacing the first liquid Ph se by other liquids being miscible with the preceding liquid phase but having a lower'critical temperature. Finally rather than to the structure after the incorporavolumes-applies to the structure prior to the in V corporation of the gas 7 opacifying substance tion. Thus in the case of aerogels it may well happen that when the gas opacifying' substance is introduced in the. form of a solutionthat on removal of thes'olvent after the introduction of the gas opaciiying substance the volume will greatly decrease. will have substantially aided in providing optimal distributlon of the gas opaciiying substance by allowing a large volum or the solution to penetrate into the pores opaciiying substances closing substance. g V a Substances which will be suitable as such gels or aerogels are, for example, the gels or aerogels allowing most or the gas to be enclosed by the enof silica, thorium oxide, zirconium oxide, the rare earth oxides, and of many other inorganic oxides and compounds which are capable of formin gels oraerogels. V 7

As an example or a gel-like substance porous glass may be mentioned such an intermediate in the preparation of Vycor"-glass. fVycor is a trade-mark by Corning Glass Works, Corning, N. Y., and designates a glass which contains about 5% B203, .5% NazO. the remainder being-silica. The production of such a glass is described in Brit. Patent 442,526 or August 8, 1934. Asia known to those skilled in the art. this type oi high silica glassis obtained glasses which have a tendency towards immiscibillty and which can be separated into two immiscible phases by a heat gives rise to the formation of 1 7 one continuous phase of a glass'rich in boric oxide treatment which and of another one which contains well above The bcric oxide phase may be leached glass may be obtained. The extreme fineness of the pores may be judged from the fact that such to be optically void under that the air inclusions ive rise to a slight bluish opaleseence only.

In view of the ease of handling, the advantageous physical properties, the heat stability, the compatibility with almost any type or enamel, and

the commercial availability the use of silica aerogelwill be preferred for the incorporation of the gas opacii'ying substance.

This invention, however. is not restricted to I described gels or gel-like substances and it will be clear to those skilled in the art how to select a suitable substance in the capillaries of which the opacifying substance can be evenly distributed and subdivided. It will be advantageous to use such substances'for the incorporation oi the gas opaciiying substance which is in the form or a fine powder to start with or -which can be readily ground or otherwise disintesrated.

In any case the gas opacifler composition ac- 75 enclosing substance and However the original structurecipitate.

cording to the present invention has to be prepared prior to its addition to the enamel mass. The gas opacifler composition according to the present invention maybe conveniently prepared by incorporating the gas opacifying substance proper in the pores, for example, of a gel-like substance in the form of a solution, suspension, dispersion, or the like so that the opacifying substance proper may thoroughly penetrate the pores of the enclosing substance. In many cases it will be advantageous to remove the solvent prior to the use of the thus formed opacifler composition. Alternatively the gas opacifier composition may be formed simultaneously with the enclosing suband the raw material capable of forming the enclosing substance may be precipitated simultaneously. Thus a solution may be mixed together and ofadye and a sol the precipitated gel will enclose the dye in its pores. The thus produced product may be dried so as to'remove the solvent retained in the pro- Various other methods for preparing the op'acifier compositions for the present invention will suggest themselves to those skilled in the art, and this invention is'not restricted to any specific method for preparing them. As mentioned above other substances than gel-like subas, for example, is

lute amount necessary,

"enclosing substance stances may be used as enclosing substances as long as they have a sufficiently porous structure so as to substantially enclose the gas opaciiying substance proper.

By using the opaciiier compositions according to the present invention, namely a gas opaciiying substance which is finely sub-divided in the pores of a highly porous substance, it becomes possible to finely distribute the opaciiier in the enamel mass with the use or a relatively small amount of theporous substance. Thus, forexample, when using one of the above mentioned inorganic gels as. the porous substance subdividing the gas cpacifying substance it will be possible to finely distribute the gas opacifier composition in the enamel mass by introducing only a fraction of a percent of the opacifier composition, The absoof course, will depend on the gas opacifying substance used and on the concentration of the gas opacifying substance in the porous material. However, in most cases it will not be'neces'sary or beneficial to use such low concentrations otthe gas opacifying substance more than 1 to 3% of the opaciiier composition calculated on the enamel frit will be necessary.

It can be readily seen, therefore, that the use of such gas opaciiier compositions'makes' it possible to finely distribute the gas proper with the use of so littleoi a foreign substance that it cannot in any way affect the physical properties of the finished enamel.

There is no definite relation between the amount of the gas opacifying substance and the and the sufficient amount of the enclosing substance will have to be determined by a few preliminary experimen In any case there should be enough of the enclosing substance available so as to enclose a prevalent portion of the gas opacilylng substance. Just by way V of example it may be mentioned that into silica aerogel 30% by weight of coal tar in a benzol solution may be introduced readily without having even approached the upper limit which can be effectively introduced. Accordingly this invention is not limited to any specific ratio between the the gas opacifying subopacifying substance stance enclosed therein and it will be understood by those skilled in the art how much of any specii'ic gas opaciiying substance can be safely introduced into the enclosing substance. Generally speaking, it may be mentioned that to be on the safe side one rather should use less than the maximum amount of the gas opacifying substance for any given amount of the enclosing substance.

With respect to the preparation of dr process enamels, the use of gas opacifying substances will be more or less unrestricted; that means almost any type of gas opacifying substance which is suitable in itself and which can be incorporated in the pores of the porous substance can be used. As a matter of fact, the variety of substances which becomes available for dry process enameling will be given greater than that which was available for the customary wet process since in the dry processthe gas opacifying substance once it is enclosed in the pores of the porous substances will have no occasion to react with its surroundings. Accordingly, any substance which is capable of evolving gases within the firing range 'oi the enamel at a time where the bubbles can be retained due to the fusion of the enamel will be suitable.

When applying the process of the present invention to the manufacture of enamels or glazes by the wet process without the use of a suspension agent, the selection of a suitable gas opacifying substance will be somewhat more restricted than for the dry process proper. In this case, it will be preferable to select gas opacifying substances which cannot be dissolved by the mill liquor so as to retain the gas opacifier within the pores of the porous material and to prevent diffusion of the gas opacifier into the mill liquor.

Thus, for example, where oils or other organic vehicles are used as a suspension liquid oil insoluble water soluble dyes may be used as opacifying agents. It will be easy for those skilled in the art to select the proper gas opacifying substance for any given suspension liquid. In this connection it may be mentioned that for gas opacifiers of a strongly reducing nature the use of lead bearing enamels or glazes should be avoided or that for such lead bearingenamels or glazes gas opacifying agents of as little reducin character as possible, preferably of oxidizing character should be used.

However the selection of a gas opacifying substance does not, form subject of the present invention and a large variety of suitable gas opacifying substances are known to those skilled in the art.

The preparation of enamels orglazes according to the present invention does not necessitate any change of the conventional procedure perhaps with the one exception that for the dry process according to this invention the opacifier will be added to the mill or otherwise mixed with the enamel frit, while heretofore opacifiers for the dry process in most cases were added to the raw batch and thus incorporated in the frit. 'In preparing dry process enamels according to the process of the present invention it often will be advantageous not to subject the gas opaciiier composition to too harsh a grinding operation since by too harsh grinding its effectiveness may be impaired. Accordingly it may be advisable to add the gas opacifier composition to the mill not at the start of the grinding process but only at a time where it still will be sufficiently distributed throughout the enamel mass. Alternatively it may be advantageous to subject the frit, the gas tions which might be used to the milling opera-.

tion in the presence of water or another liquid a precaution which will protect the gas opacifler composition from too harsh grinding. After milling the enamel mass may be dried and dusted onto the ware to be nameled in the usual way. When using a wet grinding operation for grinding and mixing the enamel mass for a dry process enamel it willbe advantageous to us such gas opacifying substances incorporated in the pores of the enclosing material which will not be dissolved out of it by the mill liquor. .Sometimes it may also be of advantage to introduce the gas opacifier composition according to the present invention into the enamel mass of a dry process enamel to ether'with a finely ground inert material such as with a finely ground portion of the enamel frit proper so as to warrant a more even distribution thereof in the enamel mass especially whenever the enamel frit proper is used in a relatively coarse grind. Such an inert addition may be used with advantage in amounts to bring the total addition of the mixture of the opacifier composition and said inert material up to about 4 to 6% calculated on the frit proper. Such an inert substance, for example a -finely ground enamel frit, may be used in a mill fineness of about 1 to 4% residue on a 200-mesh screen.

The present invention may be illustrated by the following examples:

Examples the same way as under A from 97.5 g. Sant'ocel.

and from a solution of 2.5 gms. Du Pont Oil Red in about 500 cc. benzol.

C. An opacifier composition was prepared in the same way as under A from gms. Santocel from a solution of 10 gins. Du Pont Oil Yellow in about 500 cc. benzol.

About 2.6 gms., 14 gms., and 4 gms. of the opacifier compositions under A, B, and C respectively were thoroughly mixed with 1000 grams of a leadfree cast iron dry process enamel frit. The thus formed enamel masses were dusted onto heated cast iron warein the usual way and lead to white gas opacified cast iron enamels.

This invention is not restricted by any of the above given examples but is to beunderstood in its broad scope as given in the appended claims. The term enamel mass as used in the specifi cation and in the appended claims is to be understood to comprise also glaze materials which comprise the frit proper as well as any other mill additions which may be used for any type of enamel or glaze being free from a suspension agent. The term "suspension agent as used in the description and in the appended claims is to be understood to comprise such substances as clay in aqueous enamel slips which are solid substances which substantially help the frit to be held in suspension in the enamel slurry. Accordingly the term enamel mass free from a suspension agent comprises enamel masses which are not suspended in a liquid at all as well as enamel masses in which the irit is suspended in the mill liquid directly other solid substance.

usedvin the description and in the appended claims is meant to indicate that a gas opacifying substance has been incorporated in an aerogel regardless of the physical structure of the thus resulting gas opacifler composition subsequent to the incorporation and to the possible removal 01 the solvent for the gas opaciiying substance proper. The term gel-like as used in the appended claims is to be understood to comprise without the aid 01 any I true gels and substances of a structure approxi- 2. In the method oi preparing vitreous enamels and glazes from an Enamel mass free from a suspension agent the step 1 introducing a gas opacifier composition into the enamel mass, said gas opacifier composition being a gas opaciiying substance incorporated in the pores or an inorganic gel-like substance the pores of which in-the dry state without the gas opaciiying substance being incorporated therein are substantially o! submicroscopic size.

3. In the method of preparing vitreous enamels The term "aerogel! as and glazes from an enamel mass free from a suspension agent the step oi introducing a gas opacifier composition into the enamel mass, said gas opacifier composition being a gas opacitying. substance being incorporated in the pores of an inorganic gel-like substance which in the dry state and without-the gas opacliying substance being incorporated therein has pores which are subaeeaaaa stantially oi submicrosccpic size and which contains at least about by volume air. a

4. In the method of preparing vitreous enamels and glazes iromi an enamel mass being free from 'asuspension agent the step of introducing a gas opacifying substance which is substantially incorporated in the pores of a silica aerogel into the enamel mass.

5. In the method for preparing dry process vitreous enamels the step of preparing an enamel mass at least from an enamel. irit and, a gas opaclfler composition, said gas opaciiler composition being a gas opaciiying substance incorporated in the pores of a silica aerogel.

6. In the method for preparing glass enamels the step of preparing an enamel slurry free from I a suspension agent from. at least an enamel irit. a suspension liquid, and a gas opacifier composition, said gas opacifler composition being a gas opacifying substance which is substantially insoluble in the enamel slurry and which is incorporated in a silica aerogel.

7. In the method of preparing vitreous enamels and glazes from an enamel mass being free from a suspension agent the step of introducing a gas opacifying substance which is substantially incorporated in the pores of an inorganic aerogel into the enamel mass;

8. In the method of preparing dryprocess vitreous enamels which comprises preparing an enamel mass at least from an enamel'irit and a gas opacifying substance substantially incorporated in the pores of an inorganic aerogel.

"9.-In the method of preparing glass enamels which comprises preparing an enamel slurry free from a suspension agent from at least an enamel irit, a suspension liquid and a. gas opacifler composition, said gas opacifier composition being a gas opaciiying substance which is substantially insoluble in the enamel slurry and which is substantially incorporated in an inorganic aerogel.

IGNAZ KRnmL. WERNER KREIDL. 

